Dimming method and system thereof

ABSTRACT

A dimming method for LED driving circuit is proposed. By temporary switching a pin that is originally used for the input/output of other electric signals to a high impedance node, the dimming control signal may be inputted to dim LEDs. The dimming method comprises the steps of: floating the pin every a period of time to pull the pin&#39;s voltage being equal to the dimming control signal; detecting the pin&#39;s voltage; and retrieving the dimming control signal in accordance with the detected pin&#39;s voltage and thereafter dimming the LEDs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a dimming method of LightEmitting Diode (LED). More particularly, the present invention relatesto an LED dimming method, which utilizes high impedance node forinputting a dimming control signal but without the usage of an enablepin additionally.

2. Description of the Prior Art

Nowadays, many consumer electronic products contain LCD panel, such ascell phone, PDA, and MP3 player . . . etc. The designer chooses not onlythe preferred sizes of the LCD panel, but also determines the types ofthe LCD panel and thereafter the power supply and the back-light circuitthereof in accordance with different applications. Among all kinds ofthe back-light circuit solutions, white LED is the most adopted solutionto portable consumer electronic products.

Generally, the white LED solution may be categorized into the chargepump solution and the inductance-based solution. Herein the charge pumpsolution is also called switching capacitance solution, which transfersthe power from the input terminal to the output terminal by switchingthe connections between the input terminal with a flying capacitance andbetween the flying capacitance with the output terminal. The wholeswitching process is unnecessary of any inductance device. Besides,charge pump devices occupy less volume and benefit in ease of design—thedesigner usually has no need to change the architecture of the chargepump driving circuitry but only chooses a proper capacitance deviceaccording to different demands, hence the charge pump solution is verypopular.

In many situations, the LCD panel requires dimming functions. Forexample, the user may want to brighten a cell phone's or a PDA's screenwhile under dark environment, or lower the brightness of a cell phoneand cut the back-light off after idled for a period of time. Awell-known LED dimming method is Pulse Width Modulation (PWM) dimmingtechnique, in which the LED dimming signal is modulated in PWM signal.With further classification, one of the PWM-based dimming methodsadjusts the current flowed into the LED by changing the “duty cycle” ofthe PWM dimming signal. For example, if the duty cycle of the PWMdimming signal is 60%, the current flowed into the LED will be about 60%of the max value, and the brightness of the LED will decrease 60%.Another PWM-based dimming method dims the LED by adjusting the pulsenumbers of the PWM signal. For example, the MAX1574 series white LEDcontroller of the MAXIM INC. decreases the brightness of the driving LEDwith 10% each time after the controller received a pulse of PWM signal.The above-mentioned two types of dimming methods are the most popular,even if they are a little different, but they both use PWM signal to dimLED.

Referring to FIG. 1, FIG. 1 demonstrates a conventional LED drivingcircuit based on charge pump solution. The LED driving IC 10 includes acharge pump 12, a chip enable/dimming control module 14 and a currentsink 16. Herein, the pin CP and CN of the LED driving IC 10 areconnected to a transfer-capacitor (fly capacitor) Cf, which couples theelectric current from the input terminal VIN to the output terminal VOUTfor driving LEDs. The current sink 16 controls and provides stablecurrent flowed through the LEDs while there are no external dimmingsignals. The chip enable/dimming control module 14 receives dimmingcontrol signals from the chip-enable pin EN to control the ON/OFF of thecharge pump 12 and the current sink 16, thereby achieving the mainobject of the LED driving circuit: control the current flowed throughthe LEDs and then dim the LEDs.

No matter which company's LED driving IC is, most of them need achip-enable pin as the input node for receiving dimming control signals.If the LED driving IC could receive dimming control signals without anysingle input pin (chip-enable pin), the pin's numbers could effectivelyreduced and thereafter the package size of the IC could be reduced, too.In additions, the package cost also could be decreased because of lesspins of the IC, and the LED driving circuit with the improved IC will becheaper and more competitive.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a dimming method, which is capable of receiving dimming controlsignals without any single input pin.

It is another object of the present invention to provide an LED drivingIC. The present LED driving IC does not need any single pin particularlyused to receive dimming control signals, therefore the pin's numbers,the package size and the manufacturing cost of the IC could be reducedaccordingly.

The present invention proposes a dimming method applied to driving LED,which receives dimming control signals by utilizing a pin with the othersignals in common. The present dimming method includes the steps of:every a period of time, floating the pin for an inputted dimming controlsignal pulling the voltage of the pin up and having the same potential;sampling the voltage of the pin; and restoring the dimming controlsignal according to the sampled voltage. Wherein, the dimming controlsignal is pulse width modulated signal, and the step to float the pin isaccomplished by temporally switching the pin as a high impedance node.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the invention will becomeapparent from the following detailed description of the preferredembodiments with reference to the accompanying drawings in which:

FIG. 1 demonstrates a conventional white LED driving circuit based oncharge pump solution;

FIG. 2 demonstrates an LED driving circuit with a preferred embodimentof the present invention;

FIG. 3A is the simple diagram of a charge pump;

FIG. 3B shows the timing relationship of the switching signal of acharge pump;

FIG. 4A depicts the connection relationships between the charge pump 22and the inputted dimming control signal;

FIG. 4B depicts the timing diagram of the circuitry shown in FIG. 4A;

FIG. 5A depicts an embodiment LED driving circuit with the present LEDdriving IC of the invention; and

FIG. 5B depicts another embodiment LED driving circuit with the presentLED driving IC of the invention

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some preferred embodiments of the present invention will be described indetail in the following. However, beside the detailed description, thepresent invention can also be applied widely in other embodiments andthe scope of the present invention is only limited by the appendedclaims.

The main spirit of the present invention is, instead of the ordinalchip-enable pin, the LED driving IC utilizes a pin which is set for theother signals or purposes as the input node of dimming control signals,thereby reducing the pin's numbers to have smaller package of the IC.FIG. 2 demonstrates an LED driving circuit with a preferred embodimentof the present invention. Unlike the conventional LED driving circuitshown in FIG. 1, the present LED driving IC 20 excludes a chip-enablepin EN. The dimming control signals generated from the General PurposeInput/Output (GPIO) module 28 are firstly inputted to the pin CN of thecharge pump 22, and then transferred to the chip-enable/dimming controlmodule 24. In the other words, the pin CN replaces the originalchip-enable pin EN and becomes the input node for receiving dimmingcontrol signals.

Before the detail descriptions about the present LED driving circuitshown in FIG. 2, a brief operation principle of a charge pump isdescribed in first. FIG. 3A depicts a simple diagram of a charge pump.In addition to a capacitor C, there are further two sets of switches 30and 32 that controlled by two non-overlapped switching signals φ1 and φ2(referring to FIG. 3B), wherein each set of switches are closed whilethe switching signals (φ1 or φ2) change to high level. When theswitching signal φ1 changes to high, the first set of switches 30 areclosed and the second set of switches 32 are open, therefore an externalpower source across the Vin and the GND terminal may charge thecapacitor C. When the switching signals φ2 changes to high (theswitching signal φ1 stays low), the first set of switches 30 are openedand the second set of switches 32 are closed, therefore the capacitor Cdischarges to the Vout terminal to drive loads (for example, the LEDs).Generally, frequencies of the switching signals φ1 and φ2 are scaled inMHz, and the charge pump couples current from the input terminal Vin tothe output terminal Vout by continuing switching the connections of thecapacitor C.

Referring to FIG. 4A, for conciseness of the specification, the chargepump 22 and the transfer capacitor Cf shown in FIG. 2 could beimplemented by the charge pump shown in FIG. 3A. As mentioned above,dimming control signals are inputted through the pin CN and thentransferred to the chip-enable/dimming control module 24, and thechip-enable/dimming control module 24 detects voltage of the pin CN toretrieve the dimming control signals. The resistor R is used to limitthe current outputted from the GPIO 28.

Since the GPIO 28 has smaller current-driving force than the externalpower source, the pin CN is responsive of the across voltage of thetransfer capacitor Cf as under normal operation of the charge pump 22.Hence, the chip-enable/dimming control module 24 could not detect andsample the inputted dimming control signals directly. In order to makethe voltage of the pin CN being responsive of the dimming controlsignals, a method is proposed in the present invention. Referring toFIG. 4A, by temporally closing the switches 40 and 42, the pin CN willbe floated and therefore be responsive of the inputted dimming controlsignals.

FIG. 4B depicts the timing relationship between of the circuitry shownin FIG. 4A. In side the driving IC 20, a checking signal chk isgenerated periodically, which could stop the switching signal φ1 and φ2for one or several time interval. During the period that both theswitching signal φ1 and φ2 are low, the chip-enable/dimming controlmodule 24 detects and samples the voltage of the pin CN, and thereforeretrieves the dimming control signals inputted from the GPIO 28. Thechecking signal chk could be generated by the chip-enable/dimmingcontrol module 24 or an internal clock (not shown in the drawing) of thedriving IC 20, and it's not limited in the present invention. When theswitching signals φ1 and φ2 are both low and thereafter the switches 40and 42 are both OFF, since one terminal of the transfer capacitor Cf(Pin CP) is open, so the pin CN becomes a high impedance node and isfloating. It results in the GPIO 28 being able to pull the voltage ofthe pin CN up to high (or low, according to the dimming control signal).In the other words, while the switches 40 and 42 are both OFF, thevoltage of the pin CN could be pulled up rapidly to have the samevoltage with the dimming control signals because the pin CN is currentlya high impedance, thereby making the chip-enable/dimming control module24 being able to detect and sample the dimming control signals. Withhigher frequency of the checking signal chk, the sampled dimming controlsignals are closer to the original signal waveform, so that thechip-enable/dimming control module 24 could control the current sink 26or ON/OFF of the driving IC 20 itself for dimming the LEDs accordingly.

Hence, the chip-enable pin EN could be reduced while packaging thedriving IC, and the present invention may achieves the object of theinvention for reducing the pin's numbers, decreasing the volume of theIC and the manufacturing cost. Furthermore, FIG. 5A depicts anembodiment LED driving circuit that applies the present invention whichincludes: a charge pump 22, a chip-enable/dimming control module 24 andan internal clock 58, wherein the pin CN is directly connected to thechip-enable/dimming control module 24 by reference to the transmissionline 562. The difference to the prior art is, not only the switchingsignals φ1 and φ2, the internal clock 58 also generates the checkingsignal chk for periodically stopping the switching signals φ1 and φ2.Moreover, as well as another embodiment shown in FIG. 5B, the checkingsignal chk could be generated by the chip-enable/dimming control module24 instead, as well as the previous descriptions. While applying the LEDdriving IC of FIG. 5A or 5B to the LED driving circuit shown in FIG. 2,the dimming control signals could be inputted through the pin CNdirectly.

In view of the foregoing, the main spirit of the dimming method is: themethod temporally switches a specific pin of the driving IC to highimpedance, by which the GPIO pulls the voltage of the specific pin upand therefore be responsive of the dimming control signals. Hence, itshould be understood that the present invention does not limit thespecific pin to the pin CN only, but the pin CP or anyone else is alsopossible.

In further, the present invention not only applies to the white LEDdriving circuit described in foregoing paragraphs but also the otherapplications. If the designer could find and temporally switch a pin ofan IC to high impedance, a control signal could be inputted even withouta specific pin for the control signal. Of course, the condition lies onthere is no influence on the normal operation of the other signals thatinputted/outputted through that specific pin.

Although the description discloses the preferred embodiment herein, itis not limit the spirit of the invention. It is intended that thespecification and examples to be considered as exemplary only, with atrue scope and spirit of the invention being indicated by the followingclaims.

1. A dimming method for LED driving circuit, which utilizes a pin of theintegrated circuit in common for receiving a inputting dimming controlsignal, said dimming method comprising the steps of: every a period oftime, floating the pin for an inputted dimming control signal pullingthe voltage of the pin up; sampling the voltage of the pin; andrestoring the dimming control signal according to the sampled voltage.2. The dimming method according to claim 1, wherein the dimming controlsignal is PWM signal.
 3. The dimming method according to claim 1,wherein the step of floating the pin is accomplished by switching thepin to a high impedance node.
 4. The dimming method according to claim3, wherein the step of switching the pin to a high impedance node isaccomplished by electrically opening the pin.
 5. The dimming methodaccording to claim 5, wherein the integrated circuit comprises a DCpower supply integrated circuit.
 6. An LED driving integrated circuitcomprising: a charge pump for coupling current from an input terminal toan output terminal, wherein said charge pump is further electricallyconnected with a pair of pins for connecting at least a capacitor; and acontrol module electrically connecting with one of the pair of pins forreceiving a dimming control signal, wherein said control module controlsON/OFF of the LED driving integrated circuit and the charge pump;wherein the LED driving integrated circuit further generates a checkingsignal, according to said checking signal the LED driving integratedcircuit floats one of the pair of pins to make the control moduledetecting said dimming control signal.
 7. The LED driving integratedcircuit of claim 6 further comprising a current sink for controllingcurrent flowing into at least a load connecting to said output terminal.8. The LED driving integrated circuit of claim 6, wherein said load isan LED.
 9. The LED driving integrated circuit of claim 6 furthercomprising an internal clock, wherein said internal clock generates aplurality of switching signals to the charge pump.
 10. The LED drivingintegrated circuit of claim 9, wherein said internal clock generatessaid checking signal.
 11. The LED driving integrated circuit of claim10, wherein said checking signal stops said plurality of switchingsignals with at least a period of time for floating one of the pair ofpins.
 12. The LED driving integrated circuit of claim 6, wherein saidchecking signal is generated by said control module and then transferredto said internal clock for stopping said plurality of switching signalswith at least a period of time.
 13. The LED driving integrated circuitof claim 6, wherein said checking signal is generated by said controlmodule and then transferred to said charge pump for stopping saidplurality of switching signals with at least a period of time.
 14. TheLED driving integrated circuit of claim 6, wherein said dimming controlsignal is a PWM signal.
 15. The LED driving integrated circuit of claim6, wherein said dimming control signal is outputted from GPIO.
 16. TheLED driving integrated circuit of claim 6, wherein said pair of pins arerespectively pin CP and pin CN of the LED driving integrated circuit.17. A method for inputting signals with a pin in common, said methodcomprising the steps of: floating said pin; an input signal pullingvoltage of said pin; sampling the voltage of said pin; and restoringsaid input signal according to a plurality of said voltages of said pin.18. The method according to claim 17, wherein the step of floating saidpin is accomplished by switching said pin to a high impedance node. 19.The method according to claim 18, wherein the step of switching said pinto a high impedance node is accomplished by electrically opening aterminal of said pin.